What is the amino acid in turkey that makes you tired?

The amino acid in turkey that is often associated with making people feel tired is tryptophan. Tryptophan is a precursor to serotonin, which can then be converted to melatonin, a hormone that regulates sleep. While turkey does contain tryptophan, the amount is not significantly higher than that found in other meats, and feelings of tiredness after a turkey meal are more likely due to the large quantity of food consumed rather than the tryptophan itself.